The first use of tone signaling in the PSTN network occurred in 1941 using a 10-12 button keypad. FIG. 1.
In the late 1960's or early 1970's the * and # functions were added for Centrex functions: Call Forwarding, Cancel, etc.—*72, *73 etc. FIG. 1.
With the advent of the microprocessor in the 1970's, many new adjunct systems were added on to the telephone network such as voice mail, least cost routing systems, etc. and the * and # were used more and more.
Each Touch Tone, i.e. DTMF, is comprised of 2 tones, one lower and one higher. There are 2 sets of 4 tones each that are used—
Lower Frequencies 697, 770, 852, 941 hertz
Higher Frequencies 1209, 1336, 1477, 1633 hertz
For instance, the number 1 is represented by the frequencies 697 Hz and 1209 Hz. The following table shows the DTFM frequencies and the corresponding 16 keys.
KeyFrequency #1Frequency #211697 Hz1209 Hz22697 Hz1336 Hz33697 Hz1477 Hz4A697 Hz1633 Hz54770 Hz1209 Hz65770 Hz1336 Hz76770 Hz1477 Hz8B770 Hz1633 Hz97852 Hz1209 Hz108852 Hz1336 Hz119852 Hz1477 Hz12C852 Hz1633 Hz13*941 Hz1209 Hz140941 Hz1336 Hz15#941 Hz1477 Hz16D941 Hz1633 Hz
The ABCD keys, referred to as 4th Column DTMF keys, were never introduced in the public domain. FIG. 2. Today, virtually every telephone has an electronic chip that generates these tones, but the PSTN network itself requires only the 0-9 digits to complete a dialed call.